An armrest device comprises an armrest base plate on the upper end of an armrest support rod. A support block is provided on the armrest base plate. An engagement portion of the support block engages in a slot of the armrest base plate. A plurality of grooves are formed on a projection along the slot. An elastic tongue of the support block engages with the projection to allow the armrest base plate to slide back and forth smoothly.The armrest support rod is surrounded by an intermediate tube and an armrest support tube. A turning lever in the armrest support rod is pivotally mounted to an operating lever on the upper end of the armrest support rod. A pin which passes through a hole of the turning lever engages in an opening of the intermediate tube. The opening comprises a vertical elongate portion and a plurality of horizontal engagement portions. The pin selectively engages in any one of the engagement portions to allow the armrest to be held at respective height.